Sudden cardiac death (SCD), or cardiac arrest, is the sudden, abrupt loss of heart function. It is a major health problem, causing about 335,000 deaths each year in the United States. It often occurs instantly or shortly after symptoms appear. The most common cause of SCD is cardiovascular disease, in particular, coronary heart disease. However, SCD may be caused by almost all known heart diseases including degeneration of the heart muscle or cardiac enlargement in patients with congestive heart failure (CHF). Hence, an underlying heart disease is nearly always found in victims of sudden cardiac death. Most cardiac arrests occur when the diseased heart begins to exhibit rapid and/or chaotic activity known as ventricular fibrillation.
Ventricular fibrillation is an immediately life threatening condition. One suffering from ventricular fibrillation must have immediate therapy. Such therapy may be provided by an implantable cardiac stimulation device including an implantable cardiac defibrillator (ICD).
Implantable cardiac devices are well known in the art. They may take the form of implantable defibrillators or cardioverters which treat accelerated rhythms of the heart such as fibrillation or implantable pacemakers which maintain the heart rate above a prescribed limit, such as, for example, to treat a bradycardia. Implantable cardiac devices are also known which incorporate both a pacemaker and a defibrillator (ICD).
A pacemaker may be considered to be comprised of two major components. One component is a pulse generator which generates the pacing stimulation pulses and includes the electronic circuitry and the power cell or battery. The other component is the lead, or leads, having electrodes which electrically couple the pacemaker to the heart. A lead may provide both unipolar and bipolar pacing and/or sensing electrode configurations. In the unipolar configuration, the pacing stimulation pulses are applied or intrinsic responses are sensed between a single electrode carried by the lead, in electrical contact with the desired heart chamber, and the pulse generator case. The electrode serves as the cathode (negative pole) and the case serves as the anode (positive pole). In the bipolar configuration, the pacing stimulation pulses are applied or intrinsic responses are sensed between a pair of closely spaced electrodes carried by the lead, in electrical contact with the desired heart chamber, with the most proximal electrode serving as the anode and the most distal electrode serving as the cathode.
Pacemakers deliver pacing pulses to the heart to induce a depolarization and a mechanical contraction of that chamber when the patient's own intrinsic rhythm fails. To this end, pacemakers include sensing circuits that sense cardiac activity for the detection of intrinsic cardiac events such as intrinsic atrial events (P waves) and intrinsic ventricular events (R waves). By monitoring such P waves and/or R waves, the pacemaker circuits are able to determine the intrinsic rhythm of the heart and provide stimulation pacing pulses that force atrial and/or ventricular depolarizations at appropriate times in the cardiac cycle when required to help stabilize the electrical rhythm and optimize the hemodynamics of the heart.
Pacemakers are described as single-chamber or dual-chamber systems. A single-chamber system stimulates and senses in one chamber of the heart (atrium or ventricle). A dual-chamber system stimulates and/or senses in both chambers of the heart (atrium and ventricle). Dual-chamber systems may typically be programmed to operate in either a dual-chamber mode or a single-chamber mode. Dual chamber represents coordinated atrial and ventricular activity, with the atrial contraction occurring at the appropriate amount of time before the ventricular contraction.
Recently, there has been the introduction of pacing systems that stimulate in corresponding chambers of the heart as, for example, the right ventricle (RV) and left ventricle (LV). These are termed biventricular stimulation devices and may be programmed to operate in single, dual or tri chamber modes. Future designs may include the Left Atrium (LA) and operate in quad chamber modes.
Biventricular pacing has been shown to coordinate contractions of the left and right ventricles, reduce the amount of blood flow that leaks through the mitral valve, and decreases the motion of the septal wall that separates the chambers of the heart. Such motion can affect the quantity of blood that the ventricle can pump out in a single beat. Biventricular pacing has its greatest benefit when optimally timed after the atrial contraction, and the right ventricular and left ventricular contractions are also optimally timed.
Biventricular pacing has been found to be particularly advantageous in patient's suffering from congestive heart disease because of the improved ability of the left ventricle to fully pump blood from the heart. As a result, patients are able to tolerate greater exertion, have a longer life span, and experience a higher quality of life.
Implantable cardiac defibrillators (ICD's) as previously mentioned may be incorporated with a pacemaker. They are generally electrically connected to the heart with one or more defibrillation electrodes of the lead system. One defibrillator electrode may be the conductive housing of the device. A lead of the lead system may include at least one other defibrillation electrode arranged to be positioned in the right ventricle. An arrhythmia detector detects ventricular arrhythmias, such as ventricular fibrillation. When such an arrhythmia is detected, a pulse generator delivers a defibrillating shock between the defibrillation electrode in the right ventricle and the conductive housing to terminate the fibrillation. Alternatively, the lead system of such defibrillation devices may further include another defibrillation electrode arranged to be positioned in the right atrium or superior vena cava (SVC), hereinafter referred to as the SVC electrode, which may be electrically connected to the right ventricular defibrillation electrode. In this arrangement, the defibrillating shock is delivered between the commonly connected right ventricular and SVC electrodes and the conductive housing.
As previously mentioned, ventricular fibrillation is an immediately life threatening cardiac arrhythmia. It requires immediate and effective defibrillation therapy. As a result, an ICD must be capable of providing a defibrillation shock having an output magnitude that is above the output level that is required to defibrillate the fibrillating heart chamber. This is known as the defibrillation threshold (DFT).
Since many patients at risk of SCD have an implanted cardiac stimulation device, it would be most appropriate if the device could provide some indication of SCD susceptibility. This may then be followed by notification of the susceptibility and/or a response to provide more appropriate pacing therapy and/or trending of SCD susceptibility metrics for further analysis of the patient's condition. The present invention addresses these issues.